1. Field of the Invention
The present invention relates to an intermittent switching power supply circuit which stabilizes output from a secondary output winding by controlling periodic oscillation intervals of a primary winding. More specifically, the present invention relates to an intermittent switching power supply circuit that avoids output excessive power from its secondary output winding during element malfunction.
2. Description of the Related Art
It is known that some types of intermittent switching power supply circuits are used as stabilized power supplies in AC adapters and battery chargers.
Referring now to FIG. 4, an example of a prior art intermittent switching power supply circuit 100 includes an input side and an output side. Simply stated, intermittent power supply circuit 100 enables oscillation of an oscillator when output voltage and/or current drop below thresholds, and inhibits oscillation when output voltage and/or current increases beyond slightly higher thresholds. During oscillation, power is stored in reactive (capacitive/inductive) components. The stored power is then fed as DC power to using circuits including times when oscillation is stopped.
On the input side, a direct current power supply 1, illustrated as a battery, is an unstable power supply whose output voltage varies depending on load. Direct current power supply 1 includes a high-voltage terminal 1a and a low-voltage terminal 1b. 
A transformer 2 includes a primary winding 2a and a secondary output winding 2b. One end of primary winding 2a is connected to high-voltage terminal 1a. An intermittent oscillator 3 is connected between the second terminal of primary winding 2a and low-voltage terminal 1b. 
Intermittent oscillator 3 includes the conventional elements of an oscillator, a control element, and a switching element (all not shown). The switching element alternately turns on and turns off current to primary winding 2a of transformer 2. Intermittent oscillator 3 oscillates at a fixed frequency. Intermittent oscillator 3 stops oscillation when a stop control signal, in the form of a fixed current, is received at a control terminal 3a. 
A photocoupler receiver element 39 is photocoupled to a photocoupler emitter element 35 disposed on the output side. Photocoupler receiver element 39 is connected between control terminal 3a of intermittent oscillator 3 and low-voltage terminal 1b of direct current power supply 1. When photocoupler receiver element 39 is energized by an optical signal from photocoupler emitter element 35, it applies the, required fixed current to control terminal 3a to turn off oscillation. In the absence of energization of photocoupler receiver element 39, intermittent oscillator 3 is enabled to oscillate.
A rectifier diode 4 and a smoothing capacitor 13 form a rectifying/smoothing circuit. The rectifying/smoothing circuit rectifies and smooths the AC signal from output winding 2b. Energy stored in the reactive elements are fed to the remainder of the circuit during periods of non-oscillation of intermittent oscillator 3. The smoothed result is connected to a high-voltage output line 20a and a low-voltage output line 20b. A Zener diode 14 provides an output voltage clamp to limit the voltage across smoothing capacitor 13 to the breakdown voltage of Zener diode 14.
An output monitoring circuit is disposed between output lines 20a, 20b. The output monitoring circuit includes a voltage monitoring circuit and a current monitoring circuit, described later. The output monitoring circuit energizes photocoupler emitter element 35 when the output voltage or the output current exceeds a predetermined reference voltage or current. Energization of photocoupler emitter element 35 turns of oscillation of intermittent oscillator 3, as described above.
The voltage monitoring circuit includes a pair of voltage divider resistors 30, 31 connected in series between high-voltage output line 20a and low-voltage output line 20b. An intermediate tap 32 obtains divided output voltage. The divided output voltage is connected to an inverse input terminal of an error amplifier 33a. 
A DC voltage monitoring reference power supply 34a, illustrated as a battery, is connected between an uninverted input terminal of error amplifier 33a and low-voltage output line 20b. 
Error amplifier 33a is a threshold circuit which compares the first comparison voltage input to the uninverted input terminal with the divided output voltage input to the inverted input terminal. The output of error amplifier 33a switches between its high and low values when the value of the voltage fed to its inverted input becomes more or less than the divided value fed to its uninverted input. The point at which the switch takes place is set by adjustment of the resistance values of voltage divider resistors 30, 31 or by adjustment of the first comparison voltage of monitoring reference power supply 34a. 
The output of error amplifier 33a is connected to an anode of photocoupler emitter element 35. Photocoupler emitter element 35 is connected through a resistor 36 to high-voltage output line 20a to provide it with a power supply. A resistor 37a and a capacitor 38a, connected in series, serve as an AC negative feedback element to provide stable operation of error amplifier 33a. 
The output of error amplifier 33a switches between a voltage near the voltage of low-voltage output line 20b and a voltage near the voltage of high-voltage output line 20a. When the output of error amplifier 33a is high (near the voltage of high-voltage output line 20a), insufficient voltage exists across photocoupler emitter element 35 for energization thereof. In this condition, oscillation of intermittent oscillator 3 is enabled. When the output of error amplifier 33a is low (near the voltage of low-voltage output line 20b), substantially the entire voltage difference between high-voltage line 20a and low-voltage output line 20b (minus voltage drops in resistor 36, and other elements) is available to energize photocoupler emitter 35. As a result, oscillation of intermittent oscillator 3 is inhibited.
The current monitoring circuit operates in a manner similar to the voltage monitoring circuit described above. A current detection resistor 43 is connected in series with low-voltage output line 20b. One end of current detection resistor is 43 connected to an inverted input terminal of an error amplifier 33b. The second end of current detection resistor 42 is connected to the uninverted input terminal through a current monitoring reference power supply 34b (illustrated as a battery).
The potential difference between the terminals of current detection resistor 43 is proportional to the output current flowing through low-voltage output line 20b. Error amplifier 33b compares voltage across current detection resistor 43 with a second comparison voltage from current monitoring reference power supply 34b to determine whether a predetermined reference current has been exceeded. The value of the reference current may be set by either adjusting the resistance of current detection resistor 43 or by adjusting the second comparison voltage of current monitoring reference power supply 34b. 
The output of error amplifier 33b is connected to a connection point between the output of error amplifier 33a (monitoring the output voltage), and photocoupler emitter element 35. In a manner identical to voltage error amplifier circuit 33a, current error amplifier circuit 33b energizes and de-energize photocoupler emitter element 35 in response to the current in the low-voltage output line 20b being greater or less than, respectively, the selected threshold voltage.
Serially connected resistor 37a and capacitor 38a, along with a resistor 37b and a capacitor 38b, serve as AC negative feedback elements which stabilize the operation of error amplifier 33a and error amplifier 33b, respectively.
During operation of intermittent switching power supply circuit 100, intermittent oscillator 3 oscillates at a fixed frequency. The output power from secondary output winding 2b increases as long as the output power and the output current on the secondary side remain below the predetermined reference value.
During operation, intermittent oscillator 3 is switched between oscillation and non-oscillation to maintain a roughly constant output voltage and/or current. When the load connected to high-voltage output line 20a and low-voltage output line 20b increases beyond the reference voltage, the divided voltage applied to the uninverted input terminal of error amplifier 33a increases. When the voltage at its inverted input exceeds the voltage at its uninverted input, error amplifier 33a is switched off (lowest output voltage). As a result, the voltage across photocoupler emitter 35 increases beyond the emission threshold of photocoupler emitter 35. As a consequence, photocoupler emitter 35 produces an optical signal which turns on photocoupler receiver element 39. As a consequence, intermittent oscillator 3 is turned off momentarily. When the output voltage decreases below the threshold, photocoupler emitter 35 is turned off, thereby permitting photocoupler receiver to turn off intermittent oscillator 3. During non-oscillation of intermittent oscillator 3, power stored in reactive components continues to be fed to using circuits. This procedure continues with intermittent oscillator 3 being alternately turned on and off as needed to maintain the output voltage roughly constant regardless of load.
A similar operation is performed by error amplifier 33b in response to increasing and decreasing current. As a result, as the output voltage and/or output current increase and decrease about their respective reference value, with photocoupler emitter element 35 turning on and off to continuously output a limit signal to photocoupler receiver element 39 for maintaining the voltage and current values roughly constant.
When receiving a limit signal from photocoupler emitter element 35, photocoupler receiver element 39 causes a fixed current to flow from control terminal 3a to low-voltage terminal 1b of direct current power supply 1. This constitutes a xe2x80x9cstopxe2x80x9d signal applied to control terminal 3a. As a result, intermittent oscillator 3 stops oscillation until the xe2x80x9cstopxe2x80x9d control signal is no longer received, i.e., until the flow of fixed current is stopped.
When oscillation of intermittent oscillator 3 stops, secondary output winding 2b of intermittent oscillator 3 does not receive additional output power. The voltage is maintained by energy stored in the capacitive and reactive element of the filter following transformer 2. When the output voltage and/or current decreases below their respective reference, control terminal 3a receives a xe2x80x9cgoxe2x80x9d signal (absence of current) from photoconductor receiving element 39, thereby initiating a cycle of oscillation.
As a result of the respective decrease of voltage or current below the reference value, photocoupler emitter element 35 stops emitting light, and photocoupler receiver element 39 no longer receives the limit signal. In response, intermittent oscillator 3 resumes oscillation and stable output appropriate for the load power is generated.
Referring now to FIG. 5, a graph indicates the voltage at the terminals of intermittent oscillator 3 at a load of 3 W and 6 V output voltage, and a 0.5 A output current from output lines 20a, 20b. As described above, oscillation starts and stops (is limited) below and above the 3 W load, respectively.
During operation, if output lines 20a, 20b are shorted or experience current irregularity, output current immediately exceeds the reference current and oscillation stops. In this manner, if the load is increased, an oscillation interval A is longer than a rest interval B. Conversely, if the load is reduced, oscillation interval A is shorter than rest interval B. By varying the duty cycle of intermittent oscillation, output voltage and output current are controlled in a roughly stable manner according to the load.
Referring additionally now to FIG. 6, a graph shows the voltage appearing at the terminals of intermittent oscillator 3 with no load, but with output lines 20a, 20b insulated from each other. The result is a 6 V output voltage and with 0 A output current. Oscillation remains stopped, except for widely separated short cycles of oscillation of intermittent oscillator 3 to overcome leakages in the output circuit. A very short oscillation cycle is sufficient to generate quickly sufficient induced electromotive force in secondary output winding 2b to cause the output voltage to exceed the reference voltage almost immediately.
In intermittent switching power supply circuit 100, stable output power and output current, based on the load, are provided through the output monitoring circuit. The output monitoring circuit is formed from the voltage monitoring circuit, the current monitoring circuit, and photocoupled photocoupler emitter element 35 and photocoupler receiver element 39. Unfortunately, responsiveness to irregularities is inadequate.
As an example of inadequate response, where a circuit elements breaks or malfunctions, so that the current flowing through control terminal 3a of intermittent oscillator 3 stops, oscillation continues and excessive power is developed on the secondary side. The excessive power leads to excessive heat, equipment degradation, circuit failure, and other undesirable problems.
The object of the present invention to provide an intermittent switching power supply circuit which overcomes the problems described above.
It is another object of the present invention to provide an intermittent switching power supply circuit with a second backup limit signal to protect the circuit from circuit element failure.
It is another object of the present invention to provide an intermittent switching power supply circuit formed by adding a simple protection circuit to a conventional circuit structure which eliminates excessive output power even if circuit elements break.
Briefly stated, the present invention relates to an intermittent switching power supply circuit including a protection circuit and prevents excessive output power on a secondary output side. An output power monitoring circuit monitors an output power from a rectifying/smoothing circuit. A protection circuit outputs a stop control signal to a control terminal of an intermittent oscillator when the output power monitoring circuit determines that the output power exceeds a reference power. The protection circuit and a photocoupler receiver element connect in parallel to the control terminal. Where a circuit error occurs and the photocoupler receiver element fails to output a stop control signal to the control terminal, the protection circuit provides a backup and outputs a stop control signal to stop oscillation of the intermittent oscillator element.
According to an embodiment of the present invention there is provided an intermittent switching power supply circuit, comprising: a control terminal controlling an intermittent oscillator and driving an output power, a rectifying-smoothing circuit smoothing the output power, output power monitoring means for monitoring the output power and for sending a stop control to a photocoupler emitter element when the:output power exceeds a reference power, a photocoupler receiver element, the photocoupler receiver element photocoupled to the photocoupler emitter element and controlling a control terminal of the intermittent oscillator when the output power exceeds the reference power, protection circuit means for outputting a back-up stop control signal to the control terminal when the output power monitoring means determines that the output power exceeds a reference power, the protection circuit means connecting in parallel with the photocoupler receiver element, and the protection circuit means outputting the back-up stop control signal to the control terminal and stopping the intermittent oscillator when a failure occurs even when the photocoupler receiver element fails to control the intermittent oscillator, whereby the intermittent switching power supply circuit safely controls and stabilizes the output power.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, wherein: the output power is at least one of an output voltage and an output current, whereby the intermittent switching power supply circuit is effective to stabilize the at least one of the output voltage and the output current.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, wherein: the output monitoring circuit means includes at least a feed back winding, a rectifier diode, a lag network and a pair of voltage-divider resistors, the feedback winding connecting at one end to a low-voltage terminal of a direct current power supply providing the output power, the rectifier diode connecting the other end of the feedback winding, the lag network including a first resistor and a first capacitor, an intermediate tap between the first resistor and the first capacitor, the lag network in series between an output side of the rectifier diode and the low-voltage terminal, a series connection point between the first resistor and the first capacitor, and the pair of voltage-divider resistors connecting in parallel with the first capacitor between the low-voltage terminal and a base of the series connection point.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, further comprising: a protection transistor in the protection circuit means, the protection transistor connecting the control terminal of the intermittent oscillator and the low-voltage terminal, a base of the protection transistor connecting the intermediate tap, a discharge transistor connecting between the series connection point and the low-voltage terminal, a second base on the discharge transistor, and the photocoupler receiver element connecting between a base of the second base of the discharge transistor and the control terminal.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, having an output voltage and an output current and controlling an oscillation interval of a primary winding of a transformer, comprising: a rectifying-smoothing circuit, the rectifying-smoothing circuit rectifying and smoothing the output voltage and the output current, output power monitoring means for monitoring the output voltage and the output current from the rectifying-smoothing circuit, the output power monitoring means operating a photocoupler emitter element and generating a stop output signal to a control terminal of an intermittent oscillator if either one of the output voltage and the output current exceeds a reference value, the intermittent oscillator in series with the primary winding and a low-voltage terminal of an external direct current power supply, the control terminal controlling the intermittent oscillator, a photocoupler receiver element photocoupled with the photocoupler emitter element effective to transmit the stop output signal to the control terminal, a protective circuit connecting parallel with the photocoupler receiver element to the control terminal, and the protection circuit effective to output a backup stop control signal to the control terminal when the output monitoring determines that the one of the output voltage and the output current exceeds the reference value, whereby excessive output power is minimized and stabilized even during a circuit failure.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, further comprising: a secondary winding on the transformer, the output power monitoring means on a primary side of the transformer, a feedback winding in the output power monitoring means, the feedback winding having a first end connecting to the low-voltage terminal, a rectifier diode in the output power monitoring means, the rectifier diode connecting in series to a second end of the feedback winding, a lag network in the output power monitoring means, the lag network in series with the rectifier diode, the lag network including at least a first resistor and a first capacitor, the first resistor and the first capacitor in series with an output side of the rectifier diode, a pair of voltage-divider resistors in the output power monitoring means, at least a first series connection point in series between the pair of voltage-divider resistors, and the pair of voltage-divider resistors connecting in parallel with the first capacitor between the low-voltage terminal of the external direct current power supply and the series connection point between the first resistor and the first capacitor.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, wherein: the protection circuit includes a protection transistor, the protection transistor connecting the control terminal and the low-voltage terminal of the external direct current power supply, a base on the protection transistor, and the base connecting at an intermediate tap between the pair of the voltage-divider resistors.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, further comprising: a discharge transistor, a base on the discharge transistor, the discharge transistor in series between the series connection point and the ow voltage terminal, and the photocoupler receiver element connecting the base of the discharge transistor and the control terminal of the intermittent oscillator, whereby the intermittent switching power supply circuit provides double control to the oscillation interval of the primary winding and stabilizing protection to the output voltage and the output current.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, comprising: a transformer including a secondary winding, a rectifying-smoothing circuit monitoring output from the secondary winding, output monitoring circuit means for monitoring the output power and sending a stopping signal to the transformer when the output exceeds a reference output, and protective circuit outputting means for sending a back-up stopping signal to the transformer when the output exceeds the reference output, whereby the protective circuit outputting means provides a safe and simple way to protect the intermittent switching power supply circuit from damage during element failure.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, having an output voltage and output current and controlling an oscillation interval of a primary winding, comprising: a rectifying-smoothing circuit, output power monitoring means for monitoring the output voltage and the output current from the rectifying-smoothing circuit, means for determining if the output exceeds a reference value, the means for determining in the output monitoring means, an intermittent oscillator in series with the primary winding, a control terminal controlling the intermittent oscillator controlling the oscillation interval of the primary winding, protection circuit means for controlling the control terminal when the output exceeds the reference value, and the protection circuit means effective to output a stop control signal to the control terminal when the means for determining determines that the output power exceeds the reference power, whereby excessive output power is minimized and the output power is stabilized even during a circuit failure.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, controlling a circuit oscillation interval of a primary winding and stabilizing at least one of an output voltage and an output current, comprising: an intermittent oscillator controlling the circuit oscillation interval of the primary winding and the intermittent switching power supply circuit, a control terminal on an intermittent oscillator for controlling the intermittent oscillator and for turning off the intermittent oscillator upon receipt of a control signal, output monitoring means for monitoring the output voltage and the output current and providing the control signal to the control terminal if at least one of the output voltage and the output current exceeds a reference value, and protective circuit means for providing a back up monitoring of the output voltage and the output current and providing a back up to the control signal if the at least one output voltage and current exceeds the reference value, whereby the protective circuit ensures protection of the intermittent switching power supply circuit and stability of the output voltage and the output current.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, controlling an oscillation interval of a primary winding and stabilizing an output voltage and an output current in a secondary winding comprising: a transformer includes the primary and the secondary output winding, an intermittent oscillator is connected in series between the primary winding and an external direct current power supply, the intermittent oscillator providing the oscillation interval and ceasing the providing when a stop control signal is received at a control terminal, a rectifying-smoothing circuit, the rectifying-smoothing circuit rectifying and smoothing at least one of the output voltage and the output current from the secondary output winding, an output monitoring circuit monitoring the output voltage and an output current from the rectifying-smoothing circuit and operating a photocoupler emitter element and outputting a limit signal if at least one of the output voltage and the output current exceeds a reference value, a photocoupler receiver element photocoupled with the photocoupler emitter element, the photocoupler receiver element outputting a stop control signal to the control terminal of the intermittent oscillator when the limit signal is received from the photocoupler emitter element, an output power monitoring circuit monitoring output power from the rectifying-smoothing circuit, a protective circuit outputting a control signal to the control terminal when the output monitoring circuit detects that at least one of the output voltage and the output current exceeds a reference value, and the protective circuit connecting to the control terminal in parallel with the photocoupler receiver element, whereby the output voltage and the output current are stabilized and controlled effectively.
According to another embodiment of the present invention there is provided an intermittent switching power supply circuit, wherein: the output power monitoring circuit is disposed on a primary side of aid transformer, the output power monitoring circuit includes a feedback winding connected at one end of the feed back winding to a low-voltage terminal of the external direct current power supply, the output power monitoring circuit includes a rectifier diode connected the another end of the feedback winding, the output power monitoring circuit includes a lag network, the lag network including at least a first resistor and a first capacitor connected in series between an output side of the rectifier diode and the low-voltage terminal of the direct current power supply, the output monitoring circuit includes a pair of voltage-divider resistors, the pair of voltage-divider resistors connecting in parallel with the first capacitor between the low-voltage terminal and a series connection point, the series connection point between the first resistor and the first capacitor, the protection circuit including a protection transistor, the protection transistor connecting the control terminal of the intermittent oscillator and the low-voltage terminal of the external direct current power supply, a first base in the protection transistor, the first base connecting to the intermediate tap of pair of the voltage-divider resistors, a discharge transistor, the discharge transistor connecting between the series connection point of the first resistor and the first capacitor and the low-voltage terminal of the external direct current power supply, and the photocoupler receiver element connecting between a second base of the discharge transistor and the control terminal.
The intermittent switching power supply circuit further includes: an output power monitoring circuit monitoring output power from the rectifying/smoothing circuit; and a protective circuit outputting a stop control signal to the control terminal of the intermittent oscillator when the output monitoring circuit detects that the output power exceeds a reference power. The protective circuit is connected to the control terminal in parallel with the photocoupler receiver element.
If either the output voltage or the output current exceeds the reference voltage or the reference current, the oscillation of the intermittent oscillator stops. Oscillation is resumed when the voltage or current drops to or below the reference voltage or the reference current. Thus, the output voltage and the output current remain stable around the predetermined reference voltage or reference current.
If any of the circuit elements malfunctions so that the photocoupler receiver element does not output a stop control signal to the control terminal of the intermittent oscillator even when the voltage or current exceeds the reference voltage or reference current, the increased output power exceeding a reference power is detected by the output power monitoring circuit. A stop control signal is output from the protection circuit to the control terminal of the intermittent oscillator element regardless o the output from the photocoupler receiver element. As a result, the oscillation of the intermittent oscillator stops and output power is reduced before excessive output power is generated.
A flyback voltage is generated at the feedback coil when the intermittent oscillator is oscillating. This flyback voltage charges the capacitor in the lag network by way of the rectifier. A charge voltage proportional to the secondary output power occurs across the capacitor.
During normal operation of the switching power supply, each time the photocoupler receiver element receives a limit signal, the base of the discharge transistor and the control terminal of the intermittent oscillator are made continuous. This causes a bias to be applied to the base of the discharge transistor, activating the transistor. As a result, the charge voltage in the capacitor in the lag network is discharged through the low-voltage terminal of the direct current power supply by way of the discharge transistor.
At the same time, current flows through the base of the discharge transistor and a stop control signal is sent in the form of a fixed current to the control terminal of the intermittent oscillator. This causes oscillation to stop and provides stable output voltage and output current control.
If any of the circuit elements malfunctions so that the photocoupler receiver element does not output a stop control signal to the control terminal even when the voltage or current exceeds the reference voltage or reference current, the charge voltage in the capacitor increases proportionally to the output power rather than being discharged through the discharge transistor. The charge voltage in the capacitor is divided by the divider resistors and is sent to the base of the protection transistor through the intermediate tap.
Thus, by setting up the potential generated proportionally to the reference voltage at the intermediate tap to reach the operation point of the protection transistor, the protection transistor can be activated when the output power exceeds the reference power, causing a fixed current to flow from the control terminal to the low-voltage terminal of the direct current power supply. As a result, the control terminal would receive a stop control signal regardless of the operations of the photocoupler receiver element, the oscillation of the intermittent oscillator would stop, and an excessive power output would be prevented.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.